In developmental biology there is a term to describe something that can cause deformations or growth abnormalities in a growing child, teratogenic, literally monster-making.
Any prospective parent who asks about a particular something being safe or not, any good doctor can only every respond with "it has not been determined to cause problems." Nothing is actually safe, we just don't know if something is dangerous or not. I consider this important for parents to understand fully. Doctors can be wrong and, boy, when they're wrong, they're REALLY wrong.
This really kind of drives that point home.
Ultrasound can affect fetal brain development (Via LiveScience.com)